Boolean Logic and If Statements

In Review

1. Introduction

Boolean values can be either “true” or “false”.  In computer programming we will also refer to true as “1”.  We also refer to false as “0”.  This is the fundamental number system of digital computers and also an area of philosophy, logic.  The computer includes logical functions in addition to arithmetic functions.  All the operators are shown in the table below.  We will use logic heavily with “if” statements. 

2. Comparisons

The most common use of Boolean values is in comparisons with are used heavily with “if” statements.  The following table shows the primary comparison operators.  Each of these operators will return a Boolean value (true or false().  Note that some of these operators will work with strings and numbers while others only work with numbers.

== (2 equal signs)	Equal	Strings and numbers
<>	Not equal	Strings and numbers
!=	Not equal	Strings and numbers
<	Less than	Numbers only
>	Greater than	Numbers only
<=	Less then or equal	Numbers only
>=	Greater than or equal	Numbers only

 

Below is a table with some example comparisons and their results.

Equation	Value of x
x = 1 == 1	true
x = 1 == 2	false
x = 1 <> 1	false
x = 1 <> 2	true
x = 1 < 0	false
x = 1 > 0	true
x = 1 <= 0	false
x = 1 >= 0	true
x = “hi” == “hi”	true
x = “hi” == “bye”	false
x = “hi” <> “hi”	false
x = “hi” <> “bye”	true

 

3. “If” statements

"If" statements allow us to execute one block of code if a "comparison" is true. As a simple examples let's take our random number example from before and then print out when the number is greater than 50. This is done by using the word "if" followed by a condition in parenthesis. We also have to add a colon after the parenthesis.

import random
x=0
while (x<10):
 y=random.randint(1,100) 
 print(y)
 if (y>50): print("Number is over 50")
 x+=1

If we want to have more than one line of code "inside" the "if" statement we just go to the next line and add another space:

import random

x=0
while (x<10):
 y=random.randint(1,100) 
 print(y)
 if (y>50): 
  Message="Number is over 50"
  print(Message)
 x+=1

Sometimes you'll want to do one thing when the "if" statement is true and another when it is false:

import random

x=0
while (x<10):
 y=random.randint(1,100) 
 print(y)
 if (y>50): 
  Message="Number is over 50"
 else:
  Message="Number is equal to or less than 50"
 print(Message)
 x+=1

4. Select Statements

Most languages have a "select" or "case" statement. These statements allow you to extend what we just did with "if" and "else" to any number of conditions. Python uses "if" with a special "else if" or "elif" statement to achieve the same results.

Using the same example, let's say we want to print out a different message when the value is over 75, between 50 and 75, between 25 and 50, and 25 or below.

import random

x=0
while (x<10):
 y=random.randint(1,100) 
 print(y)
 if (y>75): 
  Message="Number is over 75"
 elif (y>50):
  Message="Number is over 50 and less than or equal to 75"
 elif (y>25):
  Message="Number is over 25 and less than or equal to 50"
 else:
  Message="Number of less than or equal to 25"
 print(Message)
 x+=1

Notice that we only have to check if the number is over 50. This is because if the number is greater than 75, it will execute the code in the first "if" statement and then move to the "print()" function. Similarly, if the number is greater than 50, it will never reach the comparison for "y>25". If this is unclear, now is a perfect time to use the Wingware IDE to "walk through" the code and see what is happening.

5. Logical Operators

Logical operators operate on two Boolean values and return a Boolean value.  “And” and “Or” are the most commonly used operators and are used heavily with “if” statements.
 

 

The tables below show the values that result from two operands A and B using a Boolean operator.  The values for A and B are either 0 or 1 as shown.  The result of the combination of A and B using the specified Boolean operators are in the cells matching the column for a value of A and the row for a value of B.  An example would be “Anding” A=0 with B=0 would result in 0.

 

 

 

6. Complex Conditions

You can combine multiple boolean expressions together with the logical operates mentioned above. Let's just print out a message when a random value is greater than 25 "and" less than 75.

import random
x=0
while (x<10):
 y=random.randint(1,100) 
 print(y)
 if (y>25) and (y<75):
 Message="Number is between 25 and 75
 print(Message)
x+=1 

You can combine conditional statements with boolean expressions to make very complex decisions in your programs. We recommend you use parenthesis to keep the logic clear.

Additional Resources

Python Documentation: Conditional Expressions